Pulmonary arterial hypertension (PAH) is a progressive and fatal disease for which there is currently no cure. Pathological changes in this disease involve remodeling of the pulmonary vasculature, including marked pulmonary artery smooth muscle cell (PASMC) proliferation, leading to increased vascular resistance, right ventricular failure and death. Despite active research in PAH and improvements in patient care, there are no available therapies to hinder or reverse the vascular remodeling in this devastating disease. Numerous studies have emerged highlighting the critical importance of sphingosine kinase 1 (SphK1) and its bioactive product, sphingosine-1-phosphate (S1P), in regulating cell survival and proliferation. SphK1, a conserved, oncogenic enzyme, has notably been implicated in the progression, invasion and metastasis of several cancer types, and preclinical studies have demonstrated efficacy of SphK1 antagonism in decreasing tumor size. Exciting preliminary findings from the Machado and Natarajan labs have identified the SphK1/S1P signaling axis as a potential novel therapeutic target for vascular remodeling in PAH. We have shown that both SphK1 and S1P are elevated in PAH patients and mediate PASMC proliferation in vitro. Additionally, we determined that S1P-induced PASMC proliferation was mediated through ligation to its high- affinity receptor, S1PR2. Despite these important findings, little is currenty known about the molecular mechanisms involved in regulating SPHK1 gene expression or S1P/S1PR2-mediated PASMC proliferation in the context of PAH pathogenesis. In this comprehensive proposal, we will test the hypothesis that hypoxia and elevated levels of platelet-derived growth factor (PDGF), both pathologically found within the pulmonary vasculature in PAH, are involved in SPHK1 gene regulation and contribute to vascular remodeling. Based on our preliminary studies, we will also test whether S1P/S1PR2-induced PASMC proliferation is mediated through activation and interaction of STAT3 and HIF1a transcription factors, both known to modulate cell proliferation. The therapeutic potential of targeting S1P/S1PR2 signaling will be evaluated in established, preclinical rodent models of pulmonary hypertension. Together, these studies will provide significant mechanistic insight into the role of the SphK1/S1P signaling axis in pulmonary vascular remodeling in PAH and may lead to the development of novel therapeutic strategies in this fatal disease.